Semiconductor materials for light-emitting diode (LED) chips have a refractive index that in some cases is well above 3. The refractive index of the medium adjacent the chip in conventional LED components—usually air or plastic—is much lower. The associated sharp jump in refractive index at the interface between the LED chip and the adjacent medium makes for a comparatively small critical angle for total reflection, with the result that a large share of the electromagnetic radiation generated in the active region of the chip is reflected back into the chip by this interface.
For this reason, only a very small portion of the radiation generated in the active region is coupled directly out of the chip. In the case of conventional LED chips, the decoupling ratio for each planar decoupling surface is calculated at only a few percent.
To improve the decoupling of light from LED chips, U.S. Pat. No. 5,233,204 proposes a thick, transparent layer that is deposited epitaxially in addition to the light-generating layers and is intended to increase the percentage of light decoupled through the front of the chip.
Also known is the use of high-refractive, transparent casting compounds; however, these have failed to find widespread application because of cost considerations, among other factors. Moreover, the best casting compounds available heretofore have had a refractive index n of no more than 1.6, resulting in too large a jump at the decoupling surface of the light-emitting semiconductor component, and thus high reflection losses. In addition, the highly transparent casting compounds have undesirable chemical and mechanical properties, which also limits their large-scale industrial application.
The object of the invention is to provide an LED chip whose ratio of generated to decoupled radiation is improved over that of conventional chips and which can be mounted in conventional LED package formats. The invention is simultaneously directed to a method of fabricating such a chip that entails only slight additional technical expenditure compared to conventional methods of LED chip fabrication.
This object is accomplished by means of an LED chip and a method disclosed herein.
Advantageous improvements of the LED chip and of the method also follow.
By means of the invention, the radiation yield in an LED chip is increased by reducing the light-emitting region to an area smaller than the cross-sectional area of the LED chip. More than a 30% increase in decoupling is possible, compared to the usual implementation of the light-emitting area over the entire cross section of the LED chip.
Provided according to the invention are an LED chip comprising a radiation-emitting active region of lateral cross-sectional area FL and a radioparent window layer disposed after the radiation-emitting active region in the direction of radiation and having a refractive index nS, and which, for purposes of the decoupling of light, has a lateral cross-sectional area FC and a decoupling surface adjacent a medium having the refractive index nM, the cross-sectional area FL of the radiation-emitting active region being smaller than the cross-sectional area FC of the decoupling surface, such that the relation
      F    L    ≤                    (                              n            M                    /                      n            S                          )            2        ·          F      C      is fulfilled. Said cross-sectional area FC relates to the regions of the window layer that are available for or intended for the decoupling of light. The term “window layer” is to be understood herein both as a single layer and as a multilayer structure which as a whole performs the function of a window layer.
According to an especially preferred embodiment of the invention, it is provided that the light-emission-limiting system is implemented such that the flow of current within the LED chip, especially into and/or through the active layer, is limited to the light-emitting region. This makes it possible to limit the emission of light to a smaller region in accordance with the invention in an especially simple manner.